As is known in the art, law enforcement agencies, when authorized by a court, may use wiretapping on the telephone line to provide monitoring of incoming and/or outgoing calls to, or from, a telephone subscriber under surveillance (i.e., sometime referred to as a "target"). One technique used to provide such wiretap is to connect a so-called pen register (or dialed number recorder (DNR)) on the target telephone line. The pen register is adapted to monitor, i.e., record, transmissions on the target telephone line. The transmissions to be monitored are usually only signalling information, such as the on-hook/off-hook condition of the target telephone line as well as dialing signals, i.e., tone or pulse, as well as the telephone number of the calling party (i.e., calling number delivery (CND) information). The transmissions to be monitored may, when authorized by a court, include the voice conversation taking place on the target telephone line. Activities of interest that occur during these monitored transmissions, i.e., so-called target events, are then typically recorded in a memory, storage medium, and/or on paper for later interpretation and analysis by the law enforcement agency.
The pen register represents loop-based intercept equipment, i.e., external plant equipment that is installed in the subscriber loop. Specifically, the pen register is coupled to the target line through an interface unit. This is usually done by placing the interface unit in the crossbox, central office, listening post (i.e., the location of the pen register itself), or other point in the loop of the target telephone line and extending the subscriber loop from the interface unit to a remotely located pen register. The interface unit typically includes a decoder and amplifier. The decoder, when included, converts on-hook/off hook and rotary dc voltage change signalling pulses into DTMF tones. More particularly, when the target telephone line is on-hook, the decoder produces a constant "fourth column" DTMF "C" or "A" tone. When, from a rotary phone, the digit "3" is dialled, the decoder converts the three dc pulses produced by the rotary telephone into a series of three, "fourth column" DTMF "A" or "C" tones. These decoder-produced tones are amplified in the amplifier of the interface unit prior to transmission to the remotely located pen register. When the pen register is located at or near the coupling point to the target line, the interface may not be required to provide decoding and amplification and, in such case, the pen register records the dc signaling changes directly.
As is also known in the art, call-forward is a feature offered by many telephone companies which enables a subscriber to have calls to be received forwarded to another telephone number (i.e., to the call-forwarded telephone number) under one of a number of different conditions, such as: the called party's telephone is off-hook or busy; the called party's telephone remained unanswered after a predetermined number of rings; or, unconditionally, i.e., the subscriber has requested that all incoming calls be forwarded to the call-forwarded telephone number. Thus, with the wiretapping arrangement described above, the pen register, being located on the target telephone line of the called telephone number, and not on the telephone line of the call-forwarded telephone number, will not record the calling event to, or from, the target telephone line.